Pill bottles (the small squat ones) for holding cutting oil or tapping fluid because they're hard to tip over.
Mustard squeeze bottles for dispensing thicker pastes (I got this idea from a container of Anchorlube which came in this sort of bottle)
Yogurt containers for holding small parts, bolts, etc
Not really a use but a modification: we all use those little acid brushes for applying cutting oil and such. I was taught to trim off most of the bristles so they don't get caught on sharp edges, and to squeeze the handle in a vise so that it can fit down inside the groove made by a lathe parting tool, to apply oil as you are parting off. Also cut the handles down by about half so that they don't tip over the bottle they're being dipped in
When applying acrylic cement and other solvents, if you use regular plastic syringes and bottles, they will melt from the solvents. I use a set of glass syringes from an estate sale, and a couple of tightly- sealed glass bottles to store the stuff.
I sometimes substitute a sawn-up phone book for the usual cigarette papers (the pages are about 0.002" thick)
Not quite in the same line but...we use a common Bosch router and router table with a carbide 45 degree router bit for nearly all our chamfering in plastics and aluminum. Saves an extra setup on the manual mills. Also a bearing-guided flush-trimming bit makes glued-up plastic boxes come out quite nicely (takes care of any overhang)
When machining steel or other ferrous metals on a magnetic chuck, I cover the chuck around the part with kitchen-style plastic wrap to help when it comes time to clean up the mess (since the chuck has some residual magnetism even after turning it off, the chips want to stick to the chuck no matter what)
Michael